Authored by: Dr. James Shelton Wesley Gerrin Farm ponds in the south almost always become stratified in the summer. Stratification occurs when the water column separates into oxygen-rich water at the surface and oxygen-poor water at the bottom separated by an abrupt transition layer called the thermocline. In general, the longer a pond is stratified, the shallower the thermocline becomes, increasing the amount of oxygen-poor water at the bottom. A pond turnover occurs when a pond destratifies, causing the oxygen-poor water at the bottom to mix with the oxygen-rich water at the surface. Mixing can be triggered by abrupt changes in water temperature, intense thunderstorms with high winds, or dry periods followed by heavy rains. Turnovers are common in the fall because water temperatures can cool abruptly and frontal weather systems frequently occur, creating suitable conditions for destratification. When a pond turns over, the dissolved oxygen levels in the mixed water can be very low, creating stressful conditions for fish and other aquatic organisms. This is the most common cause of fish kills that occur in early fall. In most cases, the condition of the pond will correct itself in a matter of days. Aeration can provide an oxygenated refuge in its immediate area of influence but will not reoxygenate the entire pond. The most critical time to run aerators is overnight when oxygen levels are lowest. If fish are gasping for air at the surface following a storm or rapid temperature cooling, it is important to reduce stress to the fish as much as possible. Fish should not be fed or handled during periods of high stress. Overcrowding can also cause increased stress, so a regular harvest plan can help reduce the likelihood of large fish kills caused by oxygen depletion.
